Pneumatic Structures

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PNEUMATIC STRUCTURES

The fascination of pneumatic structures begins with the fascination of the sky.

Submitted by MAITREYI YELLAPRAGADA BA 07 ARC 007

Pneumatic structures are essentially Membrane structures which are stabilized by the pressure of compressed air and are tensile skinned, closely related to suspended structures. Inflatable structures are typically classified as Pneumatic structures and are structural forms stabilized wholly or mainly by pressure differences of gases, liquids etc materials in bulk. On solid ground, pneumatic structures had a first breakthrough as shelters for radar devices after World War II. The shelter needed to be lightweight, mobile and deployable in short time and without any metallic parts, ideal requirements for pneumatic structures.

The first experiments with pneumatic structures were undertaken during the development of hot air balloons. Brazilian priest Bartolomeu de Gusmão, in Lisbon, conducted a pioneering experiment as soon as 1709. Montgolfier brothers built an 11m diameter hot air balloon, made by linen and paper. At the same year, Jaques A. C. Charles built the first hydrogen balloon. The large rigid dirigible of the end of 19th century and beginning of the 20th century (Herzog, 1977; Forster, 1994). During the Word War II, and after the invention of nylon, pneumatics started to be used in military operations, as emergency shelters and decoys.

In 1948, Bird and his team achieved the construction of a 15m diameter pneumatic dome, the prototype for a series of large “radomes” (as they have been called) built by Birdair Structures. This pioneered, during the 1960’s, the commercial application of pneumatics, as covers for warehouses, swimming pools, sport facilities and factories. It was Frei Otto who broadened the landscape, not only of pneumatics, but of tension structures in general. Pneumatics were also part of the repertoire of Richard Buckminster Fuller. His proposal of a pneumatic dome to cover New York (1962) is a famous example of Utopian pneumatic architecture. Pneumatic structure is a membrane which carries load developed from the tensile stresses. Its stabilization is done by prestressing the membrane either by: applying an external force which pulls the membrane taut  internal pressurizing if the membrane is volume enclosing Such structures are called “pneumatic structures”. These structures can create artificial environments adaptable to human use. The pneumatic forms are bound to increase in popularity, owing to the tremendous freedom they provide to the architects in designing large free spaces within them. Principle  

Its principle is the use of relatively thin membrane supported by a pressure difference. Through increasing the inside air pressure not only the dead weight of the space envelope is balanced, but the membrane is stressed to a point where it cannot be indented by asymmetrical loading.

Properties of Pneumatic Structures i.

Light weight: The weight of the structure as compared to the area it covers is very less.   

The weight of the membrane roof, even when it is stiffened by cables, is very small. Low air pressure is sufficient to balance it. Even with spans of more than 100 meter, the weight of the structure does not exceed 3kg/square meter.

ii. 

Span : For pneumatic membrane, there is no theoretical maximum span as determined by strength, elasticity, specific weight or any other property. It is hardly possible to span a distance of over 36km. With a steel cables as they would fail because of their inability to sustain their own weight. But with pneumatics, such spans are quiet possible.

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iii. • •

Safety: Pneumatic structures are safer than any other structure. Otherwise, a proper care should be taken while establishing. Accidental circumstances are avoided as they are very light. Pneumatic structures cannot be destroyed by fire quickly and totally.

iv.

Theft: It is very safe nobody can or nothing can pass through a pneumatic structure. If an air bag is cut with a knife/ pin, a bang is produced.

v.

Quick erection and dismantling: Suitable for temporary constructions because they are as easy to dismantle and establish. 1 sq.km. of an area can be brought down in 6 hours and erected in less than 10 hours. The 4 hours difference is due to establishment of pegs etc.

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vi.

Economy: First costs for a pneumatic structure always have compared favorably with those of conventional roof structures. On a cost-per-seat basis, the advantage is even more evident. The savings come from lower construction and supporting structure costs plus overall economy of design. Architecturally, the design is very elegant and dramatic.

vii.

Good natural light: Gives good natural light as translucent/transparent plastic sheets are used to cover air bags. We can even bring the whole sun inside. There is a lot of flexibility in getting sun light (50%-80%).

TYPES OF PNEUMATIC STRUCTURES There are two primary classes of pneumatic structures Air – supported structures: An Air-Supported structure is formed by one or more layers of continuous flexible membranes anchored to the ground or to a wall so that a leak-proof seal is constituted. This airtight structure is then inflated and pressurized by the constant supply of air. On average the life of an Air Supported Facility is that between 20-25 years. The internal volume of a building air is consequently at a pressure higher than atmospheric. Air must be supplied contantly because of the continous leakage through needed openings.This puts restrictions on the amount of openings provided. However some of the advantages of this structures are:   

Their relatively low cost Their simplicity of design and fabrication The flexibility to use the facility for seasonal activities, therefore installing and uninstalling the facility throughout the year, regardless of size.

Common applications : sports stadiums, the "bubbles" used to cover tennis courts and pools, and many other temporary shelters.

Swimming pool

Storage Site

Air – inflated structures: Air cell inflatables are advanced constructions made with two layers of material with fabric formers perpendicular in between. They are self-supporting and self-erectable by means of an air fan only with no need for foundation, hardware or guy wires. The internal volume of building air remains at atmospheric pressure. The pressurized air in the pillow serves only to stablizing the load carrying membrane. The covered space is not pressurized. Advantages of air- inflated / air frame struture :  

The ability for self support The potential to support an attached structure No restrictions on the number and size of openings and design geometry.

Two major shortcomings of pneumatic structures in architecture can be named: significant form restrictions for air houses and significant load limitations for air beams. As soap bubbles demonstrate, the natural form of pneumatic structures is the sphere. Any inflated uniform elastic membrane tends to be spherical. Other basic pneumatic forms are the cylinder and the torus. Different forms can be generated by an appropriate cutting pattern of stiff fabrics and by boundary conditions. Air houses have an elongated, mainly cylindrical, shape which is familiar as the voluminous sausage impression of most inflated structures.

DROP STITCH TECHNOLOGY Drop stitch technology is in its infancy but has a great future. Drop stitch structures are fast to inflate and deflate, and it is the only way to make an inflatable surface absolutely flat and create a walking surface. The drop stitch structures have working pressures up to 1 atmosphere - much higher than any other inflatable shape. They are available in thicknesses from 5 to 50 centimetres.

FABRIC Almost all permanent fabric structures built today are entirely synthetic. The most common fibers used for the membrane are fiberglass or polyester. Fiberglass is strong and durable but deteriorates when exposed to moisture. Polyester is less expensive but it is not as strong and degrades when exposed to sunlight. Silicon rubber and Teflon are usually used to coat these materials. The fabric is not made and shipped in one piece. It is made in sheets, usually about 12' wide and varying length. The easiest and most common method of joining the fabric together is the standard lap joint. The two pieces of fabric are overlapped by three inches and Teflon FEP film is inserted between them. The joint is then heat welded together. When completed, the joint is stronger than the fabric, and completely water and air tight.

CABLES Cables are usually made from steel, because it has a low cost, availablility, and long life. Kevlar and glass fiber cables are stronger and stiffer, but are more expensive and degrade when exposed to ultraviolet light.

AIR CELL TECHNOLOGY Air cell technology marked a new era in the history of inflatable fabric engineering and pneumatic architecture. Air cell inflatables are advanced constructions (often referred to as pneumatic structures) made with two layers of material with fabric formers perpendicular in between. They are self-supporting and self-erectable by means of an air fan only with no need for foundation, hardware or guy wires. Air cell inflatable buildings (or pneumatic buildings) act as permanent structures rather than temporary ones having high torsional stiffness, which allows them to withstand wind up to 80 knots and snow load up to 140kg/m2. Inflatable buildings can support loads on the roof and walls for lighting, lifting and other cabling requirements. They have great thermal and sound insulation properties, and tolerate temperatures from -30 °C to + 70°C. Inflatable buildings fully comply with the standards applicable to pneumatic buildings - Fire Retardancy Standards (BS 7837/5438) and Anti-Fungal Standards. The life expectancy of inflatable buildings depends upon the climate in which they are installed and particularly the levels of UV light to which the pneumatic structures are exposed. An inflatable structure erected outdoors should survive for 10 years in the Tropics and for 20 years in European conditions. If the inflatables are kept indoors they will last almost indefinitely. There are almost no limitations as to design geometry for the inflatable constructions – present day facilities are capable of producing almost anything in fabric. However, the building must have a sufficient air gap to create the required rigidity, and large flat horizontal areas are to be avoided. Portable architecture brings no disruption to the site because inflatable buildings are manufactured entirely off-site and can usually be installed within a day. Pneumatic buildings and structures can be used in practically any environment and are ideally suited both for military and civil applications.

Pneumatic Structures also belong to the category of the Form Active Structures. Form active structures are those structures in which load is taken by the form or the shape of the structure. They are non-rigid, flexible matter shaped in a certain way and secured at the ends, can support itself and span space. Only tensile and compressive stresses persists. These are mainly categorized into 4 types:    

Cable Structure Arch Structure Tents Structures Pneumatic Structures

MATERIALS for Pneumatic structures:Isotropic: - These materials show the same strength and stretch in all directions. Examples are:Plastic films: - These are primarily produced from PVC, Poly ethylene, polyester, polyamide etc. Fabrics: - These may be made of glass fibers or synthetic fibers which are coated in a PVC, polyester or polyurethene film. Rubber membrane: - They are the lightest and most flexible. Metal foils: - They possess a very high gas diffusion resistance and high tensile strength. One of the major problems in the use of metal foils is in need to produce very exact cutting patterns. Anisotropic materials: - These do not show the same strength and stretch ability in all directions. They have direction oriented properties. Examples are:Woven fabrics: - They have two main direction of weave.They can be made of:. Organic fibers e.g.: - wool, cotton or silk . Mineral fibers e.g.:- glass fibers . Metal fibers e.g.:- thin steel wires . Synthetic fibers e.g.:- polyamide, polyester and polyvinyl. Gridded fabric: - These are coarse-weave made of organic mineral or synthetic fibers or metallic networks. They are particularly used where maximum light transmission and high strength is required. Synthetic rubbers: - Combination of plastic and rubber. They can take better wear and tear. They are latest and are more resistant to elongation. Plastics: - like woven fabrics. Its advantage is that they have more of tensile strength than normally manufactured plastic sheets.